Hypersonic wind tunnel



July 24, 1962 E. A. BUNT ET AL HYPERSONIC WIND TUNNEL Filed Jan.. 15, 1961 HERMAN L. OLSEN EDGAR A. BUNT INVENTORS ATTORNEYS United Staes atet 3,045,481 HYPERSONIC WIND TUNNEL Edgar A. Bunt, Kensington, and Herman L. Olsen, Derwood, Md., assignors to the United States of America as represented by the Secretary of the Navy Filed Jan. 18, 1961, Ser. No. 83,599 9 Claims. (Cl. 73-147) This invention relates generally to wind tunnels; more particularly, it relates to hypersonic wind tunnels of the continuously-operated type capable of providing realistic test conditions for comparatively long periods.

A continuously-operated hypersonic wind tunnel facility normally consists of a reservoir of high pressure static gas, a convergent-divergent nozzle section for expanding and accelerating the gas to the required velocity, a test section in which model experiments are conducted, and an exhaust section for decelerating and recompressing the gas.

In being accelerated by the nozzle section from a substantially static condition to a very high velocity in the test section the gas undergoes a severe temperature drop, the magnitude of which is dependent upon the final velocity attained. Where the final velocity is very large, say hypersonic, the rfinal temperature of the gas often is lower than that which will prevent condensation in the test section and which is required to simulate a given atmospheric condition. Thus, it becomes necessary that the temperature of the static gas be made great enough to allow [for a realistic gas temperature in the test section after the acceleration and expansion process.

For example, to expand and accelerate air flow isentropically from a static condition to a test section Mach number of 6 at a temperature of 500 =R., a stagnation temperature of 4lOO R. is required. The magnitude of this temperature is more easily comprehended when it is realized that the melting point of steel is somewhere in the vicinity of 3000 R. The problem, however, consists not so much in containing the gas as in achieving the high temperature at a high pressure.

Thus far, several methods of achieving high temperatures at correspondingly high pressures have been practiced. One method employs rocket combustors as the source of hot pressurized gases, which are then expanded by a nozzle section to hypersonic velocities. Another method employs a spark discharged through a closed pressurized chamber to provide a source of hot pressurized air. Both of these methods suffer disadvantages. In the first the temperature which can be reached by combustion is insufiicient, and in the second the operating time is limited to a few milliseconds.

Because of the disadvantages of such prior methods, the continuously-operated plasma arc has recently received attention as a heat source. Plasma are heat sources such as are utilized in this invention are well known, and hence will not be described in detail. Basically, they consist of a cooled pressurized chamber containing a pair of electrodes, which may assume any of a number of shapes. A very high electric current is supplied to the electrodes to establish an electrical discharge or are therebetween, through which a pressurized gas is caused to flow. The gas is acted upon by the arc, and then flows from the chamber through an exit orifice as a plasma discharge.

The discharge from the plasma arc heat source may not be at a sufficiently high pressure to be utilized directly in a hypersonic wind tunnel. While there appears to be no theoretical limit to the value of the operating pressure of the gas flowing through the arc chamber, practically it has been found that certain diificulties and complications arise when the plasma arc is operated at pressures above a few atamospheres. Rising pressures cause 3,045,481 Patented July 24, 1962 the arc to contract in cross-section, and at high pressures considerable radiation is emitted. Further, it is common practice to cause the points of attachment of the arc to the electrodes to move to avoid consumption of the latter. At high pressures the arc must move very rapidly, if consumption of the electrodes and resulting contamination of the gas in the tunnel is to be avoided. For these reasons it is desirable to operate the arc at a low pressure. Thus, if the plasma arc is to be utilized in a wind tunnel facility Where high pressures are needed, it is necessary to provide a means for augmenting the pressures obtainable from the plasm arc.

It has also been found that steadier arc operation is possible if an inert gas, such as nitrogen, is utilized. When such a gas is employed it is necessary that oxygen be mixed with the plasma discharge if substantially normal air is to how in the test section. In addition, depending upon the altitude to be simulated in the working section, the specific enthalpy, or total energy per unit mass, of the gas should be variable. One way of doing this without altering the main parameters of operation of the plasma arc is to dilute the gas downstream with cold gas more or less according to requirements.

The subject invention is directed to a wind tunnel facility incorporating a plasma are as a heat source, and includes an injector apparatus which will allow the addition of mass or energy in any desired proportion to the plasma discharge. The injector apparatus allows the arc heater to operate in a pressure environment lower than that required for the gas in the reservoir supplying the facility, and provides means for injecting gas to either alter the enthalpy or to make up a given composition.

It is the principal object of this invention to provide a continuously-operated type wind tunnel incorporating a plasma arc heat source and including means for augmenting the pressure of the heated gas so that realistic temperature and pressure conditions together with hypersonic flow can be produced.

A further object of the invention is to provide injector apparatus for a continuously-operated type wind tunnel facility incorporating a plasma are heat source, said apparatus being so constructed that gas may be injected into the facility downstream of the heat source whereby the composition and enthalpy of the gas flowing into the facility from the heat source may be altered.

It is also an object of the invention to provide injector apparatus for a continuously-operated type wind tunnel facility so constructed as to permit the injector to be visually observed and to be adjustable during operating thereof.

Yet another object of the invention is to provide a continuously-operated, plasma arc type wind tunnel facility so constructed as to provide a comparatively long duration hypersonic airflow having a high temperature.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing, wherein the single FIGURE is a schematic of the invention.

In the wind tunnel of the present invention a hypersonic, high temperature gas flow is obtained by intermixing a plasma discharge and a high pressure gas stream within an injector apparatus, said discharge and said stream being mixed prior to entry thereof into the nozzle section of the wind tunnel facility. The plasma discharge is created by the flow of a low pressure gas stream through a plasma are heat source. The high pressure gas stream is initially at a relatively low temperature, and flows through the injector apparatus in such a manner as to conserve the heat present therein.

The principle of operation of the injector apparatus is that of entrainment. A stream of hot gas (the discharge issuing from the plasma arc heat source) is entrained by an annular jet or comparatively cold gas directed downstream. The kinetic energy of the combined fiow is then partially recovered as pressure energy in the diffuser section of a mixing chamber, which then serves as the reservoir to feed the nozzle section of the hypersonic wind tunnel facility.

Referring now to the drawing, a plasma are heat source is indicated at 2, and an injector apparatus is generally indicated at 4. The plasma arc heater is of the type generally described hereinabove, and has an exit orifice 6 through which a plasma discharge may flow. A reservoir 8 supplies the arc heater with low pressure gas through a conduit 10 and a valve 12.

The injector apparatus 4 includes an outer shroud 14, which is sealed at both ends by bulkheads l6 and 18. A threaded nippl is sealed in bulkhead 16, which bulkhead is a part of the plasma heat source assembly 2. The exit orifice 6 extends through said threaded nipple.

Disposed within the outer shroud 14 is a mixing chamber 22 having a convergent section 24 and a divergent diffuser section 26, the forward end of said chamber passing through bulkhead 18 for connection to a flange 28 of the reduced diameter, straight-walled nozzle section 30 of the wind tunnel facility. An axially adjustable tubular element 32 is threadably attached to the nipple 20, and has its forward exterior end portion 34 conically shaped so as to fit into the convergent end 24 of mixing chamber 22. The conically shaped end portion 34 and the convergent section 24 cooperate to form the walls of a variable area annular supersonic nozzle 36, the area of which is varied by movement of the element 32 toward and away from the mixing chamber. The convergent end 24 is formed at a less acute angle than is the conical end portion 34, with respect to the axis of symmetry of element 32, suitable half-angles for these conical surfaces being 12 and 9, respectively.

The tubular element 32 includes a ring gear 38 engaging with a long pinion 40. Pinion 40 is supported at its rear end and passes through a pillow block 42 to mating bevel gears 44, one of which is attached to the end of a shaft 46 which passes through a seal to the exterior of shroud 14. Rotation of shaft 46, which may be effected by a handle 48 or by a motor (not shown), will advance or retract element 32 to vary the throat area of annular nozzle 36.

A portion of the annular nozzl 36 may be visually observed during the operation of the injector apparatus through a window 50 of suitable material, which is secured to a port 52 in the outer shroud by a clamp ring 54. A mirror 58 is mounted within the outer shroud by a sup- ,port 60, and is adjusted so that a portion of the nozzle 36 may be seen through the window. Changes in the area of annular nozzle 36 because of expansion due to heating or other causes, may thus be detected and then corrected by adjusting tubular element 32.

High pressure gas from a reservoir 62 is conducted to the outer shroud 14 through a conduit 64 and a valve 66, and is admitted into the outer shroud through a port 68. An inner shroud 70 is supported in spaced relationship to the divergent diffuser section 26 of mixing chamber 22 by a baffle ring 72, and directs the high pressure gas flow along said diffuser section to thereby regeneratively cool 'the same. The high pressure gas then passes through annular nozzle 36 to mix with the plasma discharge. The outer shroud is provided with a port 73 and a valve 75 to allow the employment of a greater amount of gas for cooling purposes than is to be injected into the are discharge, the excess gas being permitted to flow through said port and valve to the atmosphere.

In operation, a very high electric current is supplied to the electrodes of the plasma arc heater to establish an arc therein. A suitable low pressure gas, which may be an inert gas, such as nitrogen or air, is admitted to the arc chamber from reservoir 8, where it is directed to flow through the arc. A plasma discharge then flows from exit orifice 6 into tubular element 32. High pressure gas from reservoir 62, which gas may be air or some other suitable gas, is admitted to the injector apparatus 4 and flows over the diffuser section 26 and through annular nozzle 36. The cooler high pressure gas then entrains the plasma discharge, and the mixture flows into the diverging diffuser section 26, where the cooler high velocity stream and the hot entrained stream diffuse and reach a pressure greater than that in the plasma are heat source. The diffused gaseous mixture then flows into the nozzle section 30 of the wind tunnel facility.

A test section 74, suitably instrumented, receives the hypersonic flow from nozzle section 30. Thereafter the flow passes into -a diffuser section 76 designed to slow the air and increase its pressure. The diffuser 76 exhausts into a heat sink 78 which lowers the temperature of the air prior to admitting it to a vacuum pump 80. Nozzle section 30 and diffuser section 76 are cooled by circulating water through tubes 82.

While the diffuser section of the mixing chamber is cooled somewhat by the circulation of the gas flow thereover, such cooling may not be sufiicient. In such an instance auxiliary cooling means, similar to the tubes and circulating water employed on the nozzle section 30 and diffuser section 76, may be employed. Similar cooling means may also be desirable on tubular element 32.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In a wind tunnel a plasma are heat source, a wind tunnel nozzle section, an outer shroud extending between said heat source and said nozzle section, a mixing chamber disposed within said outer shroud, said chamber being in communication with said nozzle section and being spaced from said heat source, a tubular element disposed within said outer shroud between said heat source and said mixing chamber, said tubular element being movably connected at its inner end to said heat source and having its outer end portion normally spaced from said mixing chamber to thereby form an annular nozzle, said heat source being positioned to discharge plasma into said tubular element and thence into said mixing chamber, and means for supplying high pressure gas to said outer shroud, said gas flowing through said annular nozzle into said mixing chamber, in which it entrains and mixes with said plasma discharge and thence flows into said nozzle section.

2. The invention as recited in claim 1, including additionally means within said outer shroud for cooling said mixing chamber.

3. The invention as recited in claim 2, wherein said cooling means includes means within said outer shroud and cooperating with said mixing chamber for causing said high pressure gas to flow over the exterior of said chamber prior to flow thereof through said annular nozzle, whereby said gas flow acts to cool said mixing chamber.

4. In a wind tunnel, a plasma are heat source, a wind tunnel nozzle section, an outer shroud extending between said heat source and said nozzle section, a mixing chamber disposed within said outer shroud and including a convergent portion and a divergent diffuser portion, said divergent diffuser portion being in communication with said nozzle section and said convergent portion being spaced from said heat source, a tubular element disposed within said outer shroud in said space between said heat source and said mixing chamber, said tubular element being movably connected at its inner end to said heat source and having a conically shaped outer end portion, said conically shaped end portion being normally received within and spaced from said convergent portion of said mixing chamber to thereby define an annular nozzle, said heat source being positioned to discharge plasma into said tubular element and thence into said mixing chamber, means for supplying high pressure gas to said outer shroud, and means within said outer shroud and cooperating with said mixing chamber for causing said high pressure gas to flow over the exterior of said chamber, whereby to cool the same, said gas then flowing through said annular nozzle into said mixing chamber, in which it entrains and mixes with said plasma discharge and flows into said nozzle section.

5. The invention as recited in claim 4, wherein said convergent portion of said mixing chamber forms an acute angle less in value than the acute angle formed by the conical end portion of said tubular element.

6. The invention asrecited in claim 5, wherein said acute angle or said convergent portion is substantially 9, and said acute angle of said conical end portion is substantially 12.

7. The invention as recited in claim 4, wherein said last-mentioned means includes a baffle plate and an inner s-hroud, said baflie plate being secured Within said outer shroud, and said inner shroud being secured to said baffle plate and extending about said divergent diffuser portion of said mixing chamber in radially spaced relationship thereto.

8. The invention as recited in claim 4, including additionally means connected with said tubular element for moving the same toward and away from said mixing chamber.

9. The invention as recited in claim 4, including additionally means mounted within said outer shroud for permitting visual inspection of said annular nozzle.

References Cited in the tile of this patent UNITED STATES PATENTS 2,967,926 Edstrom Jan. 10, 1961 FOREIGN PATENTS 848,623 Great Britain Sept. 21, 1960 

